In the papermaking art, an aqueous suspension containing cellulosic fibers, and optional fillers and additives, is fed into a headbox which ejects the cellulosic suspension onto a forming wire. Water is drained from the cellulosic suspension to provide a wet paper web which is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced into the cellulosic suspension to facilitate drainage and increase adsorption of fine particles onto the cellulosic fibers so that they are retained with the fibers.
Silica-based sols are widely used as drainage and retention aids, usually in combination with charged organic polymers. Such additive systems are among the most efficient now in use in the papermaking industry, in particular those comprising silica-based sols which contain microgel or aggregated particles of high surface areas. Examples of silica-based sols of this type include those disclosed in U.S. Pat. Nos. 5,176,891; 5,368,833; 5,603,805 and 6,372,806 as well as International Patent Appl'n Publ. Nos. WO 98/30763; 98/56715; 00166491; 00/66492; 2005/097678 and 2005/100241.
Silica-based sols are usually shipped and stored in the form of high concentration sols. Prior to addition to the cellulosic suspension, the sols are usually diluted with water in order to improve mixing of the silica-based particles with the furnish components of the cellulosic suspension. However, depending on its origin, the dilution water used may be hard water or otherwise contain a substantial amount of salts, electrolytes, etc. which usually will affect the performance of the sols.
It would be advantageous to be able to provide silica-based sols with improved drainage and retention performance in papermaking, in particular when the sols have been diluted with hard water or dilution water containing a substantial amount of salts and electrolytes. It would also be advantageous to be able to provide a method for producing such silica-based sols. It would also be advantageous to be able to provide a papermaking process with improved drainage and retention performance.